Add cancellation test suite for deadlock prevention

Tests validate query cancellation and interrupt behavior from issue #192:
- Connection can be closed/interrupted during long-running queries
- cancel/interrupt work during VDBE execution
- cancel/interrupt work during busy handler waits
- Pool recovery after stuck queries (2 tests currently FAIL - demonstrating the bug)
- close() returns quickly even with in-flight queries
- Multiple simultaneous operations
- Busy timeout expiration behavior
- Transaction rollback during interruption

22 tests total, tagged :slow_test (take ~78s). At this commit, 20 pass and 2 fail:
- FAIL: 'pool recovers after long query interrupted on disconnect' (timeout)
- FAIL: 'pool recovers when query stuck in busy handler' (stuck waiting)

These failures demonstrate the deadlock described in #192. Subsequent commits
implement the fix that makes all 22 tests pass.

Author: mjc